The current mobile radio standard LTE has mobile radio bands defined for it that are coincident with known frequency bands and are distinguished merely by increased bandwidth. Thus, by way of example, LTE band 26 with 35 MHz of bandwidth (i.e., uplink 814 MHz-849 MHz and downlink 859 MHz-894 MHz) corresponds to band 5 defined for WCDMA with 25 MHz of bandwidth (i.e., uplink 824 MHz-849 MHz and downlink 869 MHz-894 MHz), that is 10 MHz wider in comparison therewith. The higher bandwidth decreases the transition bandwidth between the transmission band TX and the reception band RX. This results in increased demands on the frequency accuracy and therefore necessitates a filter with a very steep edge.
Furthermore, there is a need for wider bands in order to comply with new and future transmission techniques in advance. For the aforementioned LTE band 26, no filter solutions for extended bandwidths are known to date, which means that the bands are usually split and are served by two different filters.
In general, it is known practice to manufacture broadband reactance filters using branching technology and to use broadband resonators as well. However, this regularly results in the disadvantage that in a filter with increasing bandwidth the edges of the passband become shallower and thus result in excessive transition bandwidths. Such filters are then no longer suitable for mobile radio bands that have a short spacing from the adjacent band of the same transmission standard, such as the cited LTE band 26.
European Patent No. 0750394 B1 proposes modifying a branching filter, which is in the form of a ladder-type filter with SAW resonators (SAW=surface acoustic wave), such that one type of the resonators, selected from parallel and series resonators, is provided with relatively low coupling by means of a dielectric layer deposited above the resonators. This results in a filter having a higher bandwidth. At the same time, one of the two passband edges remains unaltered and meets the requirements for gradient. The other passband edge flattens out, on the other hand, and therefore exhibits an increased transition bandwidth.